Electrical and Optical Properties of Vitreous Selenium

The space-charge limited current-voltage characteristics of evaporated layers of vitreous selenium from 2.4 to 60 μ thick are described. It is shown that both an exponential and a Gaussian distribution of states will explain these characteristics. An analytic solution is derived for the case of the exponential distribution of states: $N\left(\epsilon \right)=N_0{e}^{-\frac{\epsilon }{\Delta }}$, where $N_0$ is the density of states at the valence band edge which is taken to be the origin of energy. This solution, with $N_0\sim {10}^{20}$ eV and $\Delta =0.067\mathrm{}$ eV, will account for the characteristics of all the specimens independent of thickness. The model will also account for the results of the optical absorption, electron bombardment, quenching, and positive and negative photoresponse experiments of other workers. It is not possible to obtain an analytic solution for any of these cases for a Gaussian distribution of states: $N\left(E\right)=N_0\exp \{-\frac{{(E-E_{max})}^2}{{\Delta }^2}\}$. However, it is shown that the distribution in energy of these states with $E_{max}=2.35\mathrm{}$ eV and $\Delta =0.25\mathrm{}$ eV fits the experimental results as well as does the exponential distribution. A Gaussian distribution of states would be expected theoretically in a disordered material such as vitreous selenium.